1. Field of the Invention
The present invention relates to a magnetic recording apparatus, and to a magnetic recording medium particularly used for a magnetic recording apparatus.
2. Description of Related Art
The rapidly growing IT industry increases the demand for magnetic disc units with a larger capacity than ever before. This demand stimulates the development of a highly sensitive magnetic head and a recording medium with a high S/N ratio. For improving S/N ratio of a recording medium, the recording medium should have an improved retrieving output for high-density recording. In general, a recoding medium is composed of a substrate, a first underlayer (called a seed layer), a second underlayer made of Cr-based alloy which has a body-centered cubic structure, a magnetic layer, and a carbon protective layer, which are sequentially formed one over another. The magnetic layer is formed from a Co-based alloy of hexagonal close-packed structure, such as CoCrPtB and CoCrPtTa. Improvement in retrieving output is accomplished if the magnetic layer has its (11·0) or (10·0) plane oriented approximately parallel to the substrate surface so that the c-axis (which is the easy axis of magnetization) coincides with the longitudinal direction of the magnetic layer. It is known that the crystal orientation in the magnetic layer is determined by the seed layer. There are reports saying that the (11·0) plane orients as desired if the seed layer is formed from Ta (which is disclosed in Japanese published applications JPA 4-188427, JPA8-212531 and Japanese patent JP3298893) or MgO (disclosed in Appl. Phys. Lett., vol. 67, pp. 3638-3640, December (1993)) and that the (10·0) plane orients as desired if the seed layer is formed from NiAl ally of B2 structure (disclosed in Japanese patent JP5693426). Moreover, it is known that improvement in retrieving output is achieved if the substrate surface is mechanically texturized so that anisotropic magnetization takes place in the circumferential direction. Texturizing used to be applied mostly to an Al—Mg alloy substrate coated with an NiP plating film. However, texturizing is also applied to a glass substrate to create magnetic anisotropy (disclosed in Japanese published application JPA 2001-209927).
Not only improvement in retrieving out but reduction in medium noise is also important for the medium to have a high S/N ratio. Reduction in medium noise is effectively accomplished if the magnetic layer is formed from finer particles or the magnetic layer has a smaller value of Br·t, which is the product of the remanent magnetic flux density (Br) of the magnetic layer and the thickness (t) of the magnetic layer. However, noise reduction in this manner is limited because an extreme reduction in particle size or Br·t value aggravates thermal stability. There has recently been proposed an antiferromagnetically coupled recording medium which achieves noise reduction without adverse effect on thermal stability (disclosed in Appl. Phys. Lett., vol. 77, pp. 2581-2583, October (2000) and Appl. Phys. Lett., vol. 77, pp. 3806-3808, December (2000)). This magnetic recording medium has two magnetic layers which are antiferromagnetically coupled to each other, with an Ru intermediate layer interposed between them. As compared with the recording medium of single-layer magnetic layer, it has a lower Br·t value while the magnetic layer remains thick. This is the reason why it has a reduced noise level while keeping good thermal stability.
Notwithstanding the foregoing technologies combined together, it is still impossible to realize an areal recording density in excess of 70 megabits/mm2 and it is further reduce medium noise.
Accordingly, what is needed is a recording medium with high S/N ratio, which has high recording density and at the same time reliable.